1. Field of the Invention
The present invention relates to a quadrature amplitude modulation (QAM). Specifically, this invention is a method and apparatus for generating a signal having a constant envelope in a 16 QAM system.
2. Discussion of Related Art
As information is getting important in the recent society, many communication terminals, such as a personal communication terminal and a mobile communication terminal, have been developed and spread. Since these communication terminals usually operate digitally, and transmit data through radio, they employ a digital modulation system which mixes digital data with a specified frequency signal, such as sine or cosine wave in signal transmission.
The conventional digital modulation system employed by a mobile or personal communication system is a phase shift keying (PSK) system or a quadrature PSK (QPSK) system. Under the present communication environment where frequency resource is limited, the modulation method described above cannot satisfy the requirements of high speed transmission and mass information transmission, such as image data transmission.
To solve these problems, a QAM system has been developed to obtain higher bandwidth efficiency, compared to the existing PSK or QPSK.
FIG. 1 is a block diagram of a 16 QAM system according to prior art. Serial-to-parallel converter 11 receives 4-bit data in serial and outputs them in parallel. First mapper 12 converts 2-bit data (I, I), transmitted from serial-to-parallel converter 11, into level signals corresponding to the relevant data values. Second mapper 13 converts 2-bit data (Q, Q), transmitted from serial-to-parallel converter 11, into level signals corresponding to the relevant data values. First and second mappers, 12 and 13, generate specified level signals as the following Table 1;
TABLE 1 ______________________________________ I (Q) I (Q) Output (V) ______________________________________ 0 0 -1 0 1 -3 1 0 +1 1 1 +3 ______________________________________
First mixer 14 mixes the level signal generated by first mapper 12 with signal cos.omega..sub.0 t having a predetermined frequency (.omega..sub.0). Second mixer 15 mixes the level signal generated by second mapper 13 with signal sin.omega..sub.0 t, which has the same frequency (.omega..sub.0) as the signal cos.omega..sub.0 t and a phase difference of 90.degree.. Adder 16 sums up the I channel signal, [I(t)], and the Q channel signal, [Q(t)], generated by first and second mixer, 14 and 15, respectively.
In the QAM system, 4-bit data are simultaneously modulated. When expressing a signal interval corresponding to one bit data as a Tb, each data bit is output for 4-Tb. During that time, the data of 4 bits is input to serial-to-parallel converter 11.
FIG. 2 is a graph illustrating a frequency spectrum of a signal output by the QAM system. When the period of the signal is T, the bandwidth of transmitting and receiving frequency is set to 1/T. The bandwidth of a signal according to the above QAM system becomes 1/(4Tb). Therefore, the bandwidth efficiency in the QAM system is double 1/(2Tb) in the QPSK system, or quadruple 1/Tb in the PSK system.
FIG. 3 is a block diagram of a receiver for receiving and demodulating the quadrature amplitude modulated signal through the above operations. Third mixer 31 mixes the received QAM signal with signal cos.omega..sub.0 t having a predetermined frequency (.omega..sub.0). Fourth mixer 32 mixes the received QAM signal with signal sin.omega..sub.0 t, which has the same frequency (.omega..sub.0) as the signal cos.omega..sub.0 t and a phase difference of 90.degree.. First integrator 33 integrates the signal generated by third mixer 31. Second integrator 34 integrates the signal generated by fourth mixer 32. First demapper 35 determines the level of the integrated signal generated by first integrator 33 and generates 2-bit data corresponding to the relevant levels. Second demapper 36 determines the level of the integrated signal generated by second integrator 34 and generates 2-bit data corresponding to the relevant levels.
Parallel-to-serial converter 37 receives the data bits from first and second demappers 35 and 36 in parallel and outputs them in serial. The configuration shown in FIG. 3 receives a signal, for example, cos.omega..sub.0 t-sin.omega..sub.0 t, transmitted from the QAM system. The input signal becomes cos.sup.2 .omega..sub.0 t-cos.omega..sub.0 t.multidot.sin.omega..sub.0 t after passing through mixer 31. The signal is then integrated during one symbol period (Tb) by first integrator 33. The integrated signal is expressed as .intg..sub.0.sup.Tb cos.sup.2 .omega..sub.0 tdt-.intg..sub.0.sup.Tb cos.omega..sub.0 t.multidot.sin.omega..sub.0 tdt. This is newly expressed as EQU .intg..sub.0.sup.Tb 1/2dt+.intg..sub.0.sup.Tb 1/2 cos 2.omega..sub.0 tdt-.intg..sub.0.sup.Tb cos.omega..sub.0 t.multidot.sin .omega..sub.0 tdt.[Formula1]
Values of .intg..sub.0.sup.Tb 1/2cos2.omega..sub.0 tdt and .intg..sub.0.sup.Tb cos.omega..sub.0 t.multidot.sin.omega..sub.0 tdt are "0", thus the output signal from first integrator 35 is .intg..sub.0.sup.Tb 1/2dt.
First integrator 33 and second integrator 34 generate level signals of .+-.A (A is a specified value) through the above processes. The generated signals are applied to first and second demappers 35 and 36. The demappers 35 and 36 demodulate the signals to restore original data.
However, the conventional QAM system described above has the following problems.
When the data which is input to the modulator is "0000 1011 0110" in the QAM system, signal, [I(t)], which is output by first mixer 14, is shown in FIG. 4A, and signal, [Q(t)], which is output by second mixer 15, is shown in FIG. 4B. Signal, [S(t)], which is output by adder 16, has different amplitudes according to data to be modulated, as shown in FIG. 4C.
For radio communication, since it is necessary to amplify the level of a signal forwarded through an antenna, a high power amplifier must be installed at the front stage of the antenna. Especially, since data must be transmitted between a land station and an artificial satellite, a high power amplifier must be installed at the output stage in a satellite communication system.
Usually a class C amplifier is employed as the high power amplifier to increase electric power efficiency. Since an input-to-output characteristic in the class C amplifier is non-linear, the phase is deviated in accordance with the change of amplitude signal when the amplitude of an input signal changes. This deteriorates the performance of the system. Therefore, the input signal to the high power amplifier, such as a class C amplifier, must have a constant envelope.
However, since the amplitudes of output signals change according to output data in the conventional QAM system, the QAM system cannot be used in a non-linear communication system even though it has high bandwidth efficiency.